A reader e-mailed me links to these articles (in French) with a note:
“For reasons of disproportionality, French Data Protection Authority ‘CNIL’ forbids the use of any biometric feature to control the working hours of employees:”
http://www.cnil.fr/la-cnil/actualite/article/article/lautorisation-unique-n-au-007-ne-porte-plus-sur-les-controles-dhoraires-des-salaries/
http://www.net-iris.fr/veille-juridique/actualite/30870/pas-de-biometrie-pour-controler-les-horaires-de-travail.php